The ability to differentiate between an exposed area and an unexposed area of a film or paper is essential in a photographic product. The exposed photographic product is developed by a chemical developer that affords a high amplification by the production of metallic silver resulting from the catalytic action of latent image centers formed by the exposure. The silver formed makes up the final image in black-and-white products. In color photographic products, the oxidized developer resulting from the reduction of the silver halide to metallic silver reacts with couplers to form a dye image. In a negative-working emulsion, the ability of the emulsion to differentiate an exposed area and an unexposed area of a photographic element depends on the possibility that this emulsion silver halide is reduced in exposed areas only while there is no unwanted formation of metallic silver in unexposed areas not meant to be developed during processing. However, metallic silver can be formed in unwanted areas as a result of oversensitization e.g. with gold and sulfur, or of the presence of traces of metals such as Fe, Ni, Pb, Sn, Cu, or Ni. The result of this is a density build up (or fog) in Dmin areas.
Many methods have been proposed to minimize the increase of Dmin in negative-type emulsion coatings. These methods include adding, at various stages in the preparation of a photographic emulsion, stabilizers, antifoggants, antikinking agents, latent image stabilizers, prior to coating. Examples of addenda for this purpose are disclosed in Research Disclosure, September 1994, publication No 36544, Chapter VII, page 515.
It should be noted that the differentiation between an exposed area and an unexposed area on a film or paper is not the sole criterion used to evaluate the performance of a photographic material. The photographic industry seeks to improve the speed of the emulsions without increased fogging, or even with decreased fogging, without incurring a granularity penalty.
It is, however, well-known that increasing the speed of a photographic emulsion can favor fogging and result in an increase of granularity.
Currently, most photographic materials are based on emulsions containing silver halide grains of tabular form, because it is recognized that the use of such tabular grains provides high performance silver halide photography in terms of advantages such as for instance covering power, developability, separation of native and spectral sensitivities, or speed/granularity relation. However, it is still desirable to increase the speed of the emulsion without at the same time incurring a fog and granularity penalty.